1. Field of the Invention
The present invention relates to an exposure apparatus having a plurality of substrate stages.
2. Description of the Related Art
In a photo-lithographic process for manufacturing semiconductor devices, an exposure apparatus is incorporated for transferring a reticle fine pattern onto a wafer having a photosensitive material coated thereon. An exposure apparatus in the related art includes one wafer stage so that the processes of carrying-in the wafer, alignment measurement, exposure, and carrying-out the wafer are performed in series.
However, for further improving a throughput, a twin-stage exposure apparatus including two wafer stages has been recently adopted. The twin-stage exposure apparatus includes two stations of measurement and exposure. Such a twin-stage exposure apparatus is disclosed in Japanese Patent Laid-Open No. 2000-323404, for example.
The position measurement of the wafer stage relative to the station needs a high accuracy so that the position is measured with a high-resolution laser interferometer and a reflection mirror as a target of its laser light. Each station includes the laser interferometer mounted thereon that can measure the posture of the wafer stage. The value measured by the laser interferometer may include Abbe errors due to the straightness of the laser light axis and measurement errors due to the surface configuration of the reflection mirror. These measurement errors result in the position measurement errors of the wafer stage, thus deteriorating apparatus functions.
In order to reduce such position measurement errors of the wafer stage measured by the interferometer, a correction factor and a table obtained from exposure results in advance are referred to correct the measured value. Such a technique correcting the position measurement errors of the wafer stage is disclosed in Japanese Patent Laid-Open No. 2001-59704, for example. However, the exposure apparatus shown in Japanese Patent Laid-Open No. 2001-59704 includes one wafer stage along with one station, so that the factor and the table are uniquely determined.
In the twin-stage exposure apparatus, two wafer stages reciprocate between two stations. That is, there are four combinations of the stage and the station. Thus, if the correction factor is employed, which has been uniquely established for the position measurement errors of the wafer stage in a case of one wafer stage along with one station, the residual correction error is generated, causing the positional displacement of the wafer stage. Consequently, if the position is displaced along XY-directions, overlapping errors are generated, if along Z-direction, focus errors are generated.